Sensitometers are well known in the photographic art as apparatus for making sensitometric test strips from light-sensitive material (such as photographic paper or film) to determine the resolution response from the material. A sensitometer generally comprises a light source and means for producing a graded series of exposures (also referred to as line exposures) of the light-sensitive material being tested, such as an optical wedge having a series of steps of known absorbing power.
U.S. Pat. No. 3,677,653 (Hocker) teaches a sensitometer wherein a laser and a negative lens forward of the laser are used as the means for producing a divergent coherent monochromatic light beam. When it is desired to make a sensitometric test strip of the light-sensitive material, the light-sensitive material is placed in contact with the optical wedge. The laser directs a beam through the steps of various density of the optical wedge and impinge upon the front surface of the light-sensitive material being tested. The light-sensitive material is then developed by conventional means and the characteristics of the light-sensitive material are determined by methods and apparatus already known in the art. While such an apparatus may be suitable for its intended purpose, the dimensions of the divergent laser beam change rapidly as a function of distance, thus making it difficult to control the exposure densities.
Other known methods of producing a graded series of exposures (also referred to as line exposures) of the light-sensitive material involve the displacement of a light beam (or laser beam) following reflection on a rotating prism or rotating polygon. A rapid scanning of the light beam caused by the rotating prism or polygon also produces a exposure time. U.S. Pat. No. 5,053,619 (Arimoto) relates to an optical system using rotating polygons. Rotating prisms and/or rotating polygons are expensive optomechanical components. Further, their use requires mechanical rotation at very high rotational speeds, for example, in the range of 20,000-40,000 revolutions/minute, and such high rotational speeds introduce/increase mechanical maintenance.
A known method to vary and control the laser intensity on an exposure is the use an acuosto-optical modulator (AOM). However, the use of AOMs imposes beam focusing constraints, energy density limitations, and require sophisticated electronics for control. Polygon mirrors, used in optical scanning, and AOMs are described in the "Handbook of Optics", Volume II, edited by M. Bass, McGraw-Hill, N.Y., 1995.
Accordingly, a need continues to exist for a reliable laser sensitometer which has an optical structure that is compact, optically rugged, and does not incorporate a rotating prism, rotating polygon, or an AOM.